Second-Generation Black Holes: Gravitational Waves Hint at Hierarchical Mergers

Analyses of GWTC-4.0 from multiple groups converge on a high-mass subpopulation of merging black holes consistent with hierarchical mergers, where at least one black hole is the remnant of a prior merger. Second-generation black holes should be roughly twice as massive as first-generation ones, with spins near 0.7 and random orientations; a transition around 40–45 solar masses marks the shift from low-spin to higher-spin populations. These results, obtained with diverse population models focusing on effective spins, bolster the case that dense environments produce recycled black holes and have implications for the mass gap and black-hole growth, with sharper insights expected as detectors improve.